It's common in Northern Ireland, Scotland and the north-west of England, especially, for Catholics to be called 'left-footers'. It is based on the supposed tradition whereby Protestant farm-labourers dug with the right foot on the spade, whilst Catholic ones did so with the left! Click http://wwww.britannia.org/scotland/scotsdictionary/SMALL
LETTER?.shtml here and a link will take you to a Scottish dictionary web-page. Read what it says under 'kick'.

There could conceivably be a 'sinister' connection, Henrietta, given that many Northern Irish Protestants tend to 'demonise' their Catholic neighbours, but I've never come across any explanation other than the one I offered above.